Stereotypy, flexibility and coordination: key concepts in behavioral functional morphology.

Animal movement and its muscular control are central topics in functional morphology. As experimentalists we often manipulate stimuli in a controlled setting or compare species to observe the degree of variation in movement and motor control of particular behaviors. Understanding and communicating the biological significance of these sources of variability requires a universal terminology that is presently lacking in the functional morphology literature. We suggest that 'stereotypy' be used to refer to the degree of variability observed in a behavior across trials under a given set of conditions. The ability of an organism to alter its behavior across experimental treatments is referred to as 'flexibility'. We discuss how there has been a tendency to confound the phenomenon of a behavior exhibiting low variability, which we refer to as stereotyped, with inflexibility, or the inability to alter the behavior in response to a change in stimulus. The degree of stereotypy and flexibility in a behavior need not be correlated, nor need they have a common underlying basis. Coordination, a term used to describe the relationship between different body parts during movement, can be stereotyped and can show flexibility. Stereotypy of coordination can be assessed by the strength of correlations between movements of two body parts. The influence of coordination coherence on behavioral performance has rarely been considered, and could shed light on how taxa differ in their ability to perform behaviors. We suggest definitions of the terms stereotypy, flexibility and coordination, and provide examples of how and when these terms could be used when discussing behavioral changes in functional morphology.